In adaptive communication systems, the data rate is adjusted in response to changing channel conditions. The objective of the adjustments is to maintain the ratio of energy per bit (Eb) to channel noise density (No) (Eb/No) at a constant value, commensurate with the desired bit error rate (BER). Assuming communication channel parameters remain constant, the data rate will determine the system BER. When the communications channel is relatively noise free, greater data rates are used. Conversely, as the channel becomes nosier the data rates are reduced. A change in the data rate increases or decreases the transmitted Eb, this in turn, increases or decreases the ratio of Eb/No respectively.
To respond to varying noise conditions in conventional systems, system operators typically step up power, increase coding gain, or reduce data rates. Stepping up power is generally limited due to FCC regulations and other operational and practical considerations. Increasing coding gain or reducing data rates are also options, however, they result in baud changes. Baud changes require changes to the transmitter and the receiver hardware and software.